An operational principle of a general Coriolis type mass flow meter will be described with reference to FIG. 8. The mass flow meter includes a U-shape tube 1 through which a fluid to be measured flows. A permanent magnet 2 is fixed to a bottom portion of the U-shape tube 1. Both ends of the U-shape tube 1 are fixed to a base 3. An electromagnetic drive coil 4 is disposed in a manner such that the drive coil 4 sandwiches the tube 1 and a support frame 5 tightly fixed to the base 3 for supporting the drive coil 4. The U-shape tube 1 has its vibration nodes at the portions fixed to the base 3 and a structure which looses less vibration energy like a tuning fork. Electromagnetic pickups 11, 12 are provided for detecting displacement of both arms of the U-shape tube 1. The Coriolis force is generated in the fluid flowing through the U-shape tube 1 by vibrating the tube 1 at its normal frequency .sub..omega. by an electromagnetic force acting between the drive coil 4 and the permanent magnet 2 fixed to the tube 1 and facing opposite to the drive coil 4.
A vibration mode of the U-shape tube 1 is shown in FIG. 9. The strength of the Coriolis force is proportional to the mass and velocity of the fluid flowing through the tube 1 and its direction coincides with the direction of the vector product of the moving direction of the fluid and the angular velocity at which the tube 1 is vibrated. Since the moving directions of the fluid are opposite to each other on the inlet and outlet sides of the U-shape tube 1, the Coriolis forces acts in the opposite directions to both arms of the U-shape tube 1 to generate a torsional torque in the U-shape tube 1. The torsional torque changes at the same frequency with the excitation frequency at which the tube 1 is vibrated and its amplitude value is proportional to the mass flow rate of the fluid. FIG. 10 shows the vibration mode excited by the torsional torque.
The mass flow rate of the fluid can be measured by detecting torque amplitude of the torsional vibration with the pickups 11, 12. Since, in an actual vibration of the U-shape tune 1, the torsional vibration by the Coriolis force is superimposed onto the excitation vibration by the electromagnetic drive coil 4, the vibration on the upstream side is expressed as sin (.sub..omega. t-.alpha.) and the vibration on the downstream side as sin (.sub..omega. t+.alpha.). Therefore, signals e1 and e2 detected by the pickups 11 and 12 respectively show waveforms having phase difference .sub..DELTA. t between them as illustrated in FIG. 11. The phase difference .sub..DELTA. t depends on the tube and the excitation frequency. In the case of the U-shape tube 1 with a resonance frequency of 80 Hz, a phase difference of about 120 .mu.S is obtained at the maximum flow rate of 18 kg/min. Therefore, resolution of 60 nS is required in the time measurement to guarantee the resolution of 1% in one-twentieth of this 120 .mu.S dynamic range.
The simplest method, among the various methods proposed, for measuring the phase difference counts the number of reference clock pulses corresponding to pulse width of a time difference gate pulse. FIG. 12 shows a block diagram of a circuit used for the phase difference measuring method. In FIG. 12, an upstream pickup signal 20 and a down stream pickup signal 21 are amplified by amplifiers 22 with amplification factor C and the amplified signals are digitized to binary signals by comparators 23. An exclusive OR circuit 24 applies an exclusive-OR operation to the binary signals to obtain a gate pulse 25, the pulse width of which corresponds to the time difference between the upstream and downstream pickup signals. A counter 26 measures the pulse width of the gate pulse 25 in cooperation with a reference clock 27. In this case, a reference clock pulse frequency of more than 20 MHz is required.
In practice, the U-shape tube has drawbacks such as pressure loss and difficulty of cleaning caused by its curved shape. To solve these problems, straight tube Coriolis type mass flow meters have been proposed, an example of which is shown in FIG. 13.
In FIG. 13, the flow meter includes a straight tube 15 through which the fluid to measured flows. The permanent magnet 2 is fixed at a central portion of the tube 15 and both ends of the tube 15 are fixed to the base 3. An electromagnetic drive coil 4 is disposed in a manner that the drive coil 4 sandwiches the straight tube 15 and a support frame 5 tightly fixed to the base 3 for supporting the drive coil 4. Since the straight tube shows high rigidity and is harder to bend than the U-shape tube, the above described time difference for the straight tube becomes much smaller than that for the U-shaped tube.
In the case of the straight tube 15, which usually shows a resonance frequency of 1 kHz, a phase difference of about 2 .mu.S is obtained at the maximum flow rate of 18 kg/min. Therefore, resolution of 1 nS is required in the time measurement to guarantee a resolution of 1% in one-twentieth of this 2 .mu.S dynamic range. In addition to this, if the above described measuring method which uses a counter is applied, a reference clock of 1 GHz is required which is difficult to manufacture. Even if the reference clock of 1 GHz is obtained, the counter method further requires a comparator for obtaining the time difference signal. The comparator may subject to fluctuations cause by a dead band of the input signal and it is quite difficult to obtain the precision of 1 nS. Here, intermediate levels between "1" and "0" of the signal output from the comparator is designated as a dead band, and how quickly the input signal crosses the dead band greatly affects to the fluctuations.
Because of these reasons described above, a Coriolis type mass flow meter according to the prior art is constructed as shown in FIG. 14. The mass flow meter of FIG. 14 conducts measurement and subtracts the upstream pickup signal 20 from the downstream pickup signal 21 in a difference calculation circuit (subtracter) 28 to obtain a weak phase signal with amplitude of sin .alpha. in which the phase .alpha. is 0.1 nS for the cycle of 1 mS. EQU sin(.sub..omega. t+.alpha.)-sin(.sub..omega. t-.alpha.)=2 cos .sub..omega. t.sin .alpha.
An amplifier 29 amplifies the obtained phase signal with high amplification factor and a frequency multiplier 31 obtains cos.sub..omega. t by advancing the phase of an excitation current sin .sub..omega. t of the electromagnetic drive coil by 90 degrees. A sign controller 30 switches sign of its output by outputting C.sin .alpha..cos.sub..omega. t when the cos.sub..omega. t is a positive value and C.(-cos .sub..omega. t.sin .alpha.) when the cos.sub..omega. t is a negative value. The sign controller obtains sign control timing not from waveforms, which are the objects of the sign control, but from the other signal to reduce influence of mis-operation caused by the noise.
There are many techniques for measuring the thus obtained C.sin .alpha..cos.sub..omega. t. When one measures the C.sin .alpha..cos.sub..omega. t as a time span with a microcomputer (.mu.-computer), a value C.sin .alpha. is converted in a comparator 34 to a corresponding pulse width by initially charging a capacitor with a current corresponding to the C.sin .alpha..cos.sub..omega. t during several cycles as indicated by 44 in FIG. 14, then discharging the capacitor through a constant current circuit 33 by switching a switch SW and measuring a time span between the time at which the switch SW is switched and the time at which the output of an integral circuit 32 crosses a predetermined threshold. The phase difference is then obtained by measuring the pulse width with the .mu.-computer. Since the sin .alpha. is very small, it is approximated by .alpha. in FIG. 14.
FIG. 15 shows signal waveforms of major parts of FIG. 14 when the amplitude values of the upstream and downstream detection signals are the same and the sign control signal synchronizes with cos.sub..omega. t. These signals are expressed by following equations. ##EQU1##
However, the phase measurement described above is applicable only when the amplitude values of the upstream and downstream detection signals coincide with each other and an error is caused by a difference between the amplitude values of the detection signals which is expressed by following equations, assignment of reference symbols in which are listed below.
.sub..omega. : resonance frequency of a vibrating tube
.alpha.: phase difference generated by mass flow rate
A: amplitude of downstream detection signal
B: amplitude of upstream detection signal
C: amplification factor
1. Output of the subtracter V.sub.v ##EQU2## 2. Sign control signal V.sub.S PA0 3. Output of the sign controller V.sub.C EQU V.sub.c.varies. V.sub.s.C[2A.sin .alpha. .cos.sub..omega. t-(B-A).sin(.sub..omega. t-.alpha.)] PA0 (a) Output of the subtracter V.sub.V ##EQU5## (b) Sign control signal V.sub.S 1 PA0 (c) Output of the sign controller V.sub.C 1 EQU V.sub.C 1.sub..varies. V.sub.S 1.C.[2A.sin .alpha. .cos.sub.107 t+[B-A).sin(.sub..omega. t-.alpha.] PA0 (d) Output of the integration circuit V.sub.i 1 ##EQU6##
The sign control signal VS typically corresponding to cos.sub..omega. t is obtained by frequency multiplying the input current sin .sub..omega. t of the drive coil.
V.sub.S =+1 when cos.sub..omega. t .gtoreq.0, and
V.sub.S =-1 when cos.sub..omega. t&lt;0.
4. Output of the integration circuit V.sub.i ##EQU3## where T1=0 and T2=4 .pi..
As described above, a difference occurs in the output V.sub.i of the integral circuit, as the second term of the above equation expresses, caused by the difference of the amplitude values (B.noteq.A), which further causes an integration error.
FIG. 16 shows an error expressed by a following equation when the vibration frequency of the vibrating tube is 1 kHz and generated time difference is 2 .mu.S. ##EQU4## where .intg.(B=A) expresses an integration value when the amplitude values are the same, and .intg.(B.noteq.A) an integration value when the amplitude values are different.
From FIG. 16, the error as defined above is 0.5% of the indicated value when the amplitude value difference is 1% (upstream/downstream amplitude value ratio is 101%). The amplitude value difference is reduced to some extent, for example by an AGC amplifier (automatic gain control amplifier) 35, as shown in FIG. 17, which can vary its amplification factor. However, it is difficult to realize the target error of 0.01% by installation of the AGC amplifier, because the AGC amplifier causes phase delay which varies with the amplification factor of the AGC amplifier as shown in FIG. 18.
To solve these problems, one of the inventors of the present invention proposed in the Japanese Patent Application (No. H05-110802, not yet laid open) a mass flow meter (hereinafter referred to as the proposed flow meter) which is comprised of the first pickup further comprising a permanent magnet and the first detection coil the output of which is fixed; and the second pickup further comprising an electromagnet and the second detection coil positioned opposed facing to the electromagnet, in which an input DC current to the electromagnet is controlled so as to equalize the output signal amplitude of the second detection coil the output signal amplitude of the first detection coil.
FIG. 19 is a diagram showing a configuration of the proposed flow meter. As shown in FIG. 19, the proposed flow meter is comprised of a first pickup (fixed output type pickup) 39 for detecting velocity of a fluid to be measured which includes a permanent magnet 36 and the first detection coil 37, and a second pickup (variable output type pickup) 40, the output signal amplitude of which is controlled so as to be equalized to the output signal amplitude of the first pickup and which includes an electromagnet 38 and the second detection coil 37. The output signal amplitude values of the first and second pickups are equalized by controlling the intensity of the line of magnetic force interlinking the second detection coil 37 by adjusting the input DC current to the electromagnet 38 to vary intensity of the line of magnetic force which the electromagnet 38 generates.
In FIG. 19, the permanent magnet 36 and the electromagnet 38 are fixed to the straight tube 15. The permanent magnet 36 and the electromagnet 38 are excited to vibrate in the same modes with the straight tube 15 to detect variation of the line of magnetic force which interlinks the detection coil 37. As an alternative, the permanent magnet 36 and the electromagnet 38 may be fixed to the base 3 to which the detection coil 37 is fixed. In this case, magnetic paths from the permanent magnet 36 and the electromagnet 38 may preferably be cut by the straight tube 15.
An example of a control circuit for equalizing the outputs of the fixed and variable output type pickups is shown in FIG. 20. The control circuit of FIG. 20 calculates the difference between the upstream pickup signal 20 and the downstream pickup signal 20 by the subtracter 28, and obtains a weak phase signal 2A.sin .alpha..cos.sub..omega. t, with amplitude of sin .alpha. in which the phase .alpha. is 0.1 nS for a cycle of 1 mS, and the upstream/downstream output amplitude value difference signal (B-A).cos(.sub..omega. t-.alpha.). The control circuit amplifies the phase signal with high amplification factor C by the amplifier 29 and switches the sign of the phase signal by a sign controller 50 which obtains a signal in-phase with the input signal when the upstream pickup signal sin(.sub..omega. t-.alpha.) is positive and an antiphase signal to the input signal when the upstream pickup signal sin(.sub..omega. t-.alpha.) is negative.
As a result of this sign control, only a signal which is in-phase with sin(.sub..omega. t-.alpha.), that corresponds to a term representing the output amplitude value difference between the upstream and downstream pickups, is obtained. The obtained signal is then integrated during n cycles (4 cycles in this example) by an integration circuit 51. The above described data processings are expressed by following equations.
The control signal V.sub.S 1 is obtained from the output waveform sin(.sub..omega. t-.alpha.) of the upstream detector.
V.sub.S 1=+1 when sin(.sub..omega. t-.alpha.).gtoreq.0, and
V.sub.S 1=-1 when sin(.sub..omega. t-.alpha.)&lt;0.
If the limits of integration are chosen as follows;
T1: .alpha./.sub..omega.,
T2: .alpha./.sub..omega. +4.pi., and
T3: .alpha./.sub..omega. +2.pi.,
the above V.sub.i 1 (61 in FIG. 20) is expressed by a following equation. ##EQU7##
The first term of the above equation represents an integral value when no amplitude difference remains and the second term represents an integral value when the amplitude difference remains. The first term can be neglected since the sin.sup.2 .alpha. in the first term is sufficiently smaller than 1.
An output of the integration circuit 51 is fed to an excitation current setting circuit 52. The output amplitude values of the both detectors are equalized to each other by controlling, with the output of the excitation current setting circuit 52, the electromagnet 38 of the variable output type pickup. Thus, it becomes possible to equalize the output amplitude values of the upstream and downstream pickups by the AGC technique which controls the intensity of the line of magnetic force without causing any phase delay. In the above example, one of the pickups (fixed output type pickup) is comprised of the permanent magnet 36 and the detection coil 37 and another pickup, which is controlled, is comprised of the DC electromagnet 38 and the detection coil 37. Electromagnets may be used for the both magnets. In this case, the DC excitation current of one of the electromagnet is fixed and the DC excitation current of another electromagnet is controlled.
In FIG. 19, the permanent magnet 36 and the electromagnet 38 are fixed to the straight tube 15. The permanent magnet 36 and the electromagnet 38 are excited to vibrate in the same mode with the straight tube 15 to detect variation of the line of magnetic force which interlinks the detection coil 37. As an alternative, the permanent magnet 36 and the electromagnet 38 may be fixed to the base 3 to which the detection coil 37 is fixed. In this case, a magnetic paths from the permanent magnet 36 and the electromagnet 38 may preferably be cut by the straight tube 15.
FIG. 21 shows a modification of FIG. 19. The flow meter of FIG. 21 is obtained by constructing a magnetic field generation portion of FIG. 20 with a permanent magnet 36 and a DC electromagnet 38. The permanent magnet 36 functions as a bias magnetic field and the DC electromagnet 38 functions as an output adjuster of the detection coil 37. The other functions are the same with those of FIG. 20 and their explanations are omitted.
The proposed flow meter has the following drawbacks since lead wires, for feeding the excitation current to the DC electromagnet disposed on the side of the tube, are extended from the base towards the tube: (1) possibility of breaking of wire is high; and (2) the lead wires function as a load of the vibration system to causes lowering of mechanical Q of the vibration system.
Therefore, an object of the present invention is to provide a Coriolis type mass flow meter which improves measurement precision by eliminating the mechanical Q lowering and by controlling the output amplitude values of the upstream and downstream pickups at a constant value.